OP1177/OP2177/OP4177

Rev. E | Page 15 of 24

(5V/DIV)

OPA277

Figure 52. Gain Linearity

INPUT OVERVOLTAGE PROTECTION

When input voltages exceed the positive or negative supply

voltage, most amplifiers require external resistors to protect

them from damage.

The OPx177 has internal protective circuitry that allows voltages as

high as 2.5 V beyond the supplies to be applied at the input of

either terminal without any harmful effects.

Use an additional resistor in series with the inputs if the voltage

exceeds the supplies by more than 2.5 V. The value of the resistor

can be determined from the formula

(

)

mA

5

500

≤

Ω

+

−

S

S

IN

R

V

V

With the OPx177 low input offset current of <1 nA maximum,

placing a 5 kΩ resistor in series with both inputs adds less than

5 μV to input offset voltage and has a negligible impact on the

overall noise performance of the circuit.

5 kΩ protects the inputs to more than 27 V beyond either supply.

Refer to the THD + Noise section for additional information on

noise vs. source resistance.

OUTPUT PHASE REVERSAL

Phase reversal is defined as a change of polarity in the amplifier

transfer function. Many operational amplifiers exhibit phase

reversal when the voltage applied to the input is greater than the

maximum common-mode voltage. In some instances, this can

cause permanent damage to the amplifier. In feedback loops, it

can result in system lockups or equipment damage. The OPx177

is immune to phase reversal problems even at input voltages

beyond the supplies.

TIME (400µs/DIV)

Figure 53. No Phase Reversal

SETTLING TIME

Settling time is defined as the time it takes an amplifier output

to reach and remain within a percentage of its final value after

application of an input pulse. It is especially important in measure-

ment and control circuits in which amplifiers buffer ADC inputs

or DAC outputs.

To minimize settling time in amplifier circuits, use proper

bypassing of power supplies and an appropriate choice of circuit

components. Resistors should be metal film types, because they

have less stray capacitance and inductance than their wire-wound

counterparts. Capacitors should be polystyrene or polycarbonate

types to minimize dielectric absorption.

The leads from the power supply should be kept as short as

possible to minimize capacitance and inductance. The OPx177

has a settling time of about 45 μs to 0.01% (1 mV) with a 10 V

step applied to the input in a noninverting unity gain.

OVERLOAD RECOVERY TIME

Overload recovery is defined as the time it takes the output

voltage of an amplifier to recover from a saturated condition to

its linear response region. A common example is one in which

the output voltage demanded by the transfer function of the

circuit lies beyond the maximum output voltage capability of

the amplifier. A 10 V input applied to an amplifier in a closed-

loop gain of 2 demands an output voltage of 20 V. This is beyond

the output voltage range of the OPx177 when operating at ±15 V

supplies and forces the output into saturation.

Recovery time is important in many applications, particularly

where the operational amplifier must amplify small signals in

the presence of large transient voltages.